1. Field of the Invention
The present invention relates to a white electroluminescent polymer and an organic electroluminescent device using the same. More particularly, the present invention relates to a white electroluminescent polymer including a 3,3′-bicarbazyl group incorporated into the main chain of the polymer, and an organic electroluminescent device using the same.
2. Description of Related Art
Electroluminescence phenomenon using organic materials was initially reported by Pope et al. in 1963. Tang et al. reported in 1987 a brightness and high-efficiency electroluminescent device emitting green light formed using tris(8-quinolinolato)aluminum (Alq3), a pigment having a π-conjugated structure, and that has a multi-layered structure, quantum efficiency of 1% at 10 V or less, and a brightness of 1000 cd/m2. Since then, much research and studies have been actively conducted on Alq3 all over the world.
On the other hand, low-molecular weight materials are particularly advantageous over polymers for use as electroluminescent materials because they can be prepared via simple synthesis routes. In addition, these low molecular weight materials can be used to synthesize various luminescent materials capable of emitting light of the three primary colors, (i.e., red, green and blue lights,) in the visible light region through a proper design of molecules. Today, the level of electroluminescence (EL) display technology using low molecular weight materials has achieved, in the case of passive matrix type full color displays, sizes of 10″, and, in the case of active matrix type displays employing a thin film transistor technique, about 15.1″ (Samsung SDI Co., Ltd., Republic of Korea).
However, there are two major obstacles that need to be overcome: (i) luminescence efficiency; and (ii) lifetime of electroluminescent devices. In the case of monochromatic devices, a long lifetime can be achieved to some degree. For example, the lifetime of a blue-emitting device is about 20,000 hrs (Idemitsu Kosan, Japan), a green-emitting device 50,000 hrs (Eastman Kodak, U.S.A.) and a red-emitting device 20,000 hrs (Eastman Kodak, U.S.A.), respectively. However, in practice, the blue- and red-emitting devices still need to be improved in terms of luminescence efficiency, and problems still need to be solved when using them in full color devices. With respect to the luminescence efficiency, luminescence properties of a high efficiency and a high brightness can be expected only when using a multilayered system that includes a buffer layer, a hole transporting layer (HTL), an electron transporting layer (ETL), and a hole blocking layer (HBL).
High molecular weight materials, polymers usually have a drive voltage of 2 to 3 V less than low molecular weight materials. They also are advantageous because they can be used on a flexible substrate by applying them to the substrate using spin coating or roll coating techniques. Furthermore, in respect of performance, the high molecular weight materials do not fall behind the low molecular weight materials. However, there still are some defects (e.g., they fail to achieve particular results) when the high molecular weight materials are applied to the full color method, except for ink jet printing, and that they are difficult to adapt to a mass production system. Moreover, one of the primary defects of these high molecular weight materials is that blue light emitting materials with a long lifetime and a high color purity are not achieved.
In many ways white electroluminescent devices having high luminescence efficiency are important subjects of research and intensive studies in the development of automotive dome lights or full color displays combining backlight and color filter in Liquid Crystal Displays (LCDs). Particularly, a combination of the white light and the color filter in polymer LED is considered to have great potential for application in the full color display. In general, methods for inducing white luminescence are roughly divided into two categories. The first method is to dope a host material used in the emitting layer with guest materials (luminescent dyes). In this system, luminescence occurs when energy is transferred from the host material having a large energy band gap to the dopant having a relatively small energy band gap, or when some carriers are trapped in the dopant site. In the former case, since an incomplete energy transfer takes place, separate luminescence of the host material and the dopant can be observed. For multilayered devices based on low molecular weight materials, at least 3 to 4 layers typically are used to produce white luminescence. On the other hand, for multilayered devices based on polymeric materials, two types of polymeric materials can be used that are respectively doped with pigments having low molecular weights or blended with another polymeric material having a wavelength longer than that of the matrix in a proper proportion. However, it is not easy to find the optimal conditions for such blending or doping. In particular, one of the most serious problems is deterioration of the device performance such as color purity and durability due to phase separation, which can be caused by driving of the device for a long time. With respect to the white electroluminescence, color purity is one of the most important requirements to be fulfilled and moreover, stability of color purity is considered to be the most important above all other things. Therefore, it also is imperative to attain a polymeric system having morphological stability so as not to undergo the phase separation.
The second method is only applicable to the multilayered devices based on low molecular weight materials. As claimed in this method, a new layer blocking certain carriers, i.e., holes or electrons, is disposed between the hole transporting layer and electron transporting layer as an exciton recombination zone, whereby the hole transporting layer and the electron transporting layer separately emit their respective lights. In this case, the concentration of the dopant used, and the thickness of respective layers should be properly adjusted so that pure white luminescence can be obtained.
Generally, in electroluminescent devices using organic materials, the driving voltage of fabricated devices depends in part on the ease of hole injection from an anode into an emitting layer, and the luminescence efficiency depends in part on the effectiveness of electron injection from a cathode into an emitting layer. The hole injection into the luminescence layer is readily performed, and thus relatively easy, whereas the electron injection into the luminescence layer is relatively difficult. Furthermore, the hole mobility is several tens of times higher than the electron mobility. Thus, a major charge carrier in electroluminescent devices is holes. A luminescence mechanism of normal electroluminescent devices involves holes injected from an anode, and electrons injected from a cathode. The holes and electrons are recombined in an emitting layer to form singlet excitons. The excitons then radioactively decay, upon which light having a wavelength corresponding to a band gap of the used luminescent materials is emitted. At this point, luminescence efficiency also is determined. When the amounts of the injected holes and electrons are balanced, the optimal luminescence efficiency is obtained. Thus, the mobility and density of the holes and electrons should be balanced so that these two carriers can be transported at a similar rate to each other.
However, in general, since the holes are transported more favorably and easily, disproportion in transport rates between the carriers can occur, which in turn causes a reduction in the luminescence efficiency. Therefore, for the purpose of offsetting the above imbalance, luminescent devices typically are formed in a multilayered fashion to have an electron transport layer having high electron mobility.
Carbazole has been widely used as a photoconductive, photovoltaic, photorefractive and electroactive material due to its strong fluorescent intensity and high hole transporting capability. In particular, poly(N-vinylcarbazole) (PVK), in which a carbazole molecule is incorporated as a side chain, still is used as a hole transport material in the organic electroluminescence field. Such a dimeric carbazole has an improved thermal stability and electrochemical stability, when compared to monomeric carbazole derivatives. In addition, the dimeric carbazole exhibits low oxidation potential by extension of conjugation and thus is expected to be useful as a material for hole transport layers or luminescence layers.
The description herein of certain disadvantages of known devices, systems, methods, and apparatus, is not intended to limit the scope of the present invention to devices, systems, methods, and apparatus that do not include these materials. Indeed, certain embodiments of the present invention may employ the known devices, systems, methods, and apparatus, without suffering from their previously known disadvantages.